Turbine



Aug.25,1970 p .s-f-EINACK ETAL 3,525,575

' TURBINEv Filed May 16, 1968 INVENTORS: Karl Sfeinack Rober Ehrich Waler Zrner Jrgen Freyer mex/ Unted States Patent O1 lice Int. C1. Fid 5/06 U.S. Cl. 415-170 2 Claims ABSTRACT 0F THE DISCLOSURE A quick-starting, axial-flow multiple-stage turbine in which the seal between the rotating and stationary parts is spaced outwardly from the surface of the turbine shaft, this being done by means of an annular shoulder carried by a rotor disc, which shoulder coacts with an inwardly directed seal carried by the stator. In this way, the rotor shaft itself is less sensitive to the thermal stresses which would take place if the seal were located at the surface of the shaft. According to a further feature, the rotor shaft is hollow and has a thinner wall in the hotter region of the turbine than in the cooler region thereof.

BACKGROUND OF THE INVENTION The present invention relates to an axial-flow turbine and, more particularly, to a steam turbine which can be quick-started for use as an auxiliary to meet the need for additional power during peak loads.

For economical and operational reasons when peak loads occur it is necessary to provide quick-starting and stopping times. This allows the power requirementsstemming from the occurrence of unexpectedly heavy loads to be met immediately. The time it takes to stop or start a steam turbine is an important factor in determining the severity of the thermal stresses which occur in the structural components thereof, particularly in the casing and rotor components.

Thermal stresses are developed in the structural components of a steam turbine as a result of sudden changes in the temperature of the Working medium, which results from the working medium coming into contact with the structural components. Moreover, the structural components themselves develop temperature gradients between the portion of the component which came into direct contact with the working medium, and portions of the component which are relatively distant from the working medium. Thus, thermal stresses that occur are directly related to the thickness of the walls of the turbine components. The thicker the Walls are, the greater will be the gradient in temperature within the component. From this it follows that thermal stresses are relatively small when the walls of the components are not very thick and when such walls are heated on both sides. In contrast, the thermal stresses are relatively high when the walls are thick and are heated only on one side. For example, thermal stresses occur when a thick-walled turbine casing is suddenly heated in its interior by a working medium such as steam. Under such conditions, extremely high thermal stresses are developed in the turbine casing.

The problem of thermal stresses must be taken into consideration when designing steam turbines. A particularly critical operational time for steam turbines is dur- 3,525,575 Patented Aug. 25, 1970 ing starting and stopping, especially, from a relatively cold state. When a steam turbine is started, thermal stresses which are created in the structural components must not exceed certain critical values. Certain structural components, for example, the casing and rotor, are more critical than others and in designing the steam turbine particular attention must be paid to providing that these components are capable of sustaining the thermal stresses that occur.

Several approaches toy the problem of how to reduce the thermal stresses that occur in the casing of a steam turbine are known to the art. For example, the application of ange heating reduces the starting time of the steam turbine.

Another approach known to the art is to construct the turbine casing of several parts, for example, in halves in a high-pressure machine. This approach concentrates mainly on providing a pressure gradient in the component in order to reduce the static stress on joints, screws and the like. This approach has an added benefit in that it provides a more economical use of working materials. More particularly, those parts of the casing which come into contact with steam in the highest temperature ranges can be made of relatively expensive high-heat-resistant materials, While those parts of the casing which are contacted by steam at a generally lower range of temperatures can be made of relatively less expensive and less heat-hesistant materials. Constructing a turbine casing of several parts also affects the occurrence of temperature gradients in the casing. By such a cnstruction, the thermal stresses that occur inthe casing can be minimized.

The parts of the rotor which are located in the early stages of a turbine are subjected to relatively greater thermal stresses, since temperatures in the rotor core lag considerably behind temperatures nearer the rotor periphery. An examination of cracking due to thermal stresses in steam turbine rotors has led to the discovery that those parts of the rotor which have grooves or recesses are particularly susceptible to thermal cracking. Such grooves are provided, for instance, in external seal mountings, in interstage seals and mountings therefor, as well as, mounting grooves for rotor blades or buckets provided on the rotor discs. The rotors of steam turbines are particularly susceptible to thermal cracking, especially auxiliary turbines used during peak loads. Such steam turbines are started and stopped quite frequently and experience great variations in stress.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an improved auxiliary steam turbine which can be started or stopped in a shorter time, as peak loads occur, by minimizing thermal stresses and the detrimental eects caused thereby.

The present invention accomplishes this object by locating interstage seals, i.e., seals provided between consecutive turbine stages, radially away from the rotor shaft. According to this arrangement, a sealing means is provided having an inwardly directed circumferential surface which is spaced from the rotor shaft. The rotor shaft has rotor discs located in the turbine stages. A circumferential surface is provided on an annular shoulder means carried by and arranged alongside of at least one of the rotor discs on the rotor shaft and axially aligned with the circumferential surface of the sealing means. The circumferential surfaces of the sealing means and annular shoulder coact with each other to seal the space between consecutive turbine stages, radially away from the rotor the rotor shaft with grooves for a sealing surface which will correspond to the sealing ring of the stator assembly. Thermal stresses which would concentrate at such grooves in the rotor shaft and would affect the operation of the turbine are thereby avoided.

By a further aspect of the invention the fact that the temperature of the working medium is different along different axial regions of the turbine is taken advantage of to reduce thermal stresses in the rotor shaft. The rotor shaft is constructed as a hollow member, having a wall thickness which is thinner in hotter regions of the turbine than in cooler regions thereof.

BRIEF DESCRIPTION OF THE DRAWING The single figure is a partial sectional view of a steam turbine engine arrangement, according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, there is shown a longitudinal section of a steam turbine according to the present invention. The steam turbine includes a casing formed of an outer member 2 and inner members 3 and 4 and a rotatable shaft 1 arranged in the casing having rotor discs 13 located in a plurality of turbine stages. A working medium, for example steam, is delivered to the upstream end of the turbine at a nozzle ring 8 by way of a steam conduit 5. The steam conduit 5 is surrounded by a valve housing 7. Piston ring bushings 6 mounted between the steam conduit 5 and housing 7 locate the conduit 5 centrally with respect to the housing 7 and allow the steam conduit 5 to expand when heated. The steam delivered to nozzle ring 8 by conduit 5 flows downstream therefrom to rotor blades or buckets mounted on a rotor disc 9 of the first turbine stage.

The steam then fiows along the inner surface of the steam-tight inner casing member 4 and is divided into two streams of relatively equal size which are oppositely directed with respect to each other. The inner casing member 4, in addition to being steam-tight, is expandable when heated with reference to the inner casing member 3 which is provided radially outwardly therefrom. The inner casing member 4 is provided the capability of expanding when heated by means of a piston ring bushing 6 provided in the casing member 4 mounting means, which is connected to casing member 3.

The turbine early stage or inlet stator vanes have support members 10 which extend from casing members 2, 3 and 4, respectively. The support members 1()` are provided sealing means in the form of a ring 11 on their radially inner peripheries. The sealing means 11 includes an inwardly directed circumferential surface which is spaced from shaft 1 and is arranged between two consecutive turbine stages.

As further shown in the figure, annular shoulder means 12 are carried by and arranged axially alongside of rotor discs 13. The annular shoulder means 12 have outer circumferential surfaces which are in axial alignment with the circumferential surface of the sealing means 11. By this arrangement, the circumferential surfaces of the sealing means 11 and of the shoulder means 12 coact with each other to seal the space between the two consecutive turbine stages.

The rotor shaft 1 is constructed as a hollow member having a wall thickness which varies from a relatively lesser thickness in hotter regions to a relatively greater thickness in cooler regions of the turbine.

The interstage seal arrangement provided by the present invention makes it possible to have a rotor shaft with a continuous, uninterrupted, smooth surface which is free of grooves and which is therefore much less susceptible to thermal stresses and their detrimental effects. The labyrinths, characteristic of interstage seals of steam turbines, are thus removed from the rotor shaft surface and, instead, are provided remote from the rotor shaft 1, on the shoulders 12 of discs 13 where detrimental thermal stresses do not develop. This arrangement virtually eliminates the limitations placed on the starting time required for steam turbines, by forces exerted on the rotor shaft as a result of unduly high thermal stresses. Moreover, the susceptibility of the rotor shaft to cracking, as a result of being continuously subjected to thermal stresses during operation, is reduced. Hence, greater variations in load conditions are made possible without cracking developing in the rotor during starting and stopping operations.

To have the support members 10 with sealing means 11 extend radially inwardly to a point spaced from rotor shaft 1 results in the further advantage that the axial dimensions of the support members 10, as a whole, can also be reduced. Hence the entire length of the rotor and therefore the steam turbine can be reduced.

By constructing the rotor shaft as a hollow member having a wall thickness which varies from a relatively lesser thickness in hotter regions to a relatively greater thickness in cooler regions of the turbine, the thermal stresses that result from starting and stopping operations are kept at relatively equal magnitudes in all temperature regions of the turbine. It is possible to construct the rotor shaft of varying thickness due to relatively smaller centrifugal forces being realized in the vicinity of the early turbine stages than in later stages thereof. This is due to relatively shorter rotor blade lengths at the hotter early stages than in the cooler later stages of the turbine. If the rotor shaft were constructed to have a constant thickness or cross section, during starting and stopping operations, the greatest thermal stresses would occur in the steam inlet or early stage end of the turbine. In starting and stopping operations, steam temperatures do not change quite as much in the later turbine stages, as they do in the early turbine stages. Therefore, thermal stresses that occur in the later stages are considerably smaller than those occurring in the early stages of the turbine.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

We claim:

1. A quick-starting, axial-flow multiple-stage turbine, comprising, in combination:

(a) a casing defining a plurality of stages;

(b) a shaft rotatably arranged in said casing and having rotor discs located in the stages, said rotor discs and said shaft being machined from one piece of material;

(c) support members located in the stages, alternating with said discs, said support members extending radially toward the outer circumferential surface of said shaft;

(d) sealing means arranged between two consecutive stages and fastened to the radially inward circumference of said support member, said sealing means having an inwardly directed circumferential surface which is spaced from the outer circumferential surface of said shaft; and Y (e) annular shoulder means carried by and arranged axially alongside of at least one of said discs and having an outer circumferential surface which is in axial alignment with said circumferential surface of said sealing means, said circumferential surfaces of said sealing means and of said shoulder means coacting with each other for sealing the space between the two consecutive stages.

2. A turbine as defined in claim 1 wherein the temperature of the working medium is different along different axial regions of the turbine, and wherein said shaft is honow and has a thinner wan in the hotter region of 2,762,559 the turbine than in the cooler region thereof. 2,786,625 2,823,891 References Cited 3,018,085 UNITED STATES PATENTS 5 gg-ggg 2,367,134 1/1945 Mierley. 2,649,315 8/1953 Ipsen.

6 Faught. Kent et al. Baker et al 253-149 X Welsh. Leavitt. Welch et al.

EVERETTE A. POWELL, JR., Primary Examiner 

